Wastewater Treatment

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What is bulking

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Bulking occurs when the sludge fails to settle in the clarifier. This leads to solids in the effluent, which is called bulking and foaming sludges. It is caused by the excess growth of filamentous bacteria, which causes an increase in the suspended solids levels.

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Why is it a problem

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This is a problem because it decreases the efficiency of the plant, as floc forming biomass is lost from the system because it is no longer recycled.

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Strategies used to control bulking-1

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Chlorination: Kills everything in the plant. This should be avoided if possible, since it takes months for the process plant to get back to normal again.

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Strategies used to control bulking-2

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Selective chlorination: controlled levels of chlorination of the return activated sludge (i.e. add chlorine to sludge travelling from the settler back to the reactor)

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Strategies used to control bulking-3

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Raise dissolved oxygen levels in aeration basin. Large flocs may limit O2 diffusion

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Strategies used to control bulking-4

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Raise the Food to Microorganism ratio. This can be achieved by changing the volume of reactors, and is raised by the use of selector tanks.

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Importance of analysis

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It is also important to do analysis of the sludge to determine which filamentous types are present and why they are present. Knowing this can help to understand and characterise the growth of a problem filament and hence control bulking

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Processes of wastewater treatment-1

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First is preliminary treatment, which aims to remove large materials such as rubblish. This is achieved by the use of bar screens and grit chambers.

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Processes of wastewater treatment-2

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Next is the primary treatment, which further clarifies the water via physical and chemical processes.

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Processes of wastewater treatment-3

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The most important step is the secondary treatment, activated sludge, which is an aerobic, suspended growth treatment. In the aeration tank, air is injected to provide oxygen for the metabolic breakdown of the waste, while bacteria and other organisms facilitate in the separation and settling of biomass. After time under aeration, the mixed liquor (containing bacterial solids) passes to a secondary clarifier, where bactieral solids are settled and clear, purified effluent is produced. Most of the bacterial solids are recycled to the head of the aeration tank to work again on new incoming water.

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Processes of wastewater treatment-4

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Tertiary treatment is not as widely used, but various methods such has microfiltration can be used to further purify the water.